James Handy was in dozens of films and TV shows over his decades-long career ... and he was committed to one more project before he was brutally slain this week at his girlfriend's home ... but it sounds like fans will never see his last work.

Heidi Anderson-Swan, writer and producer of the film "Kickback, V" tells TMZ ... James was one of the sweetest people she's met in Hollywood -- he helped her a few times on projects when she was breaking into the industry -- and she had to have him in her film.

Sadly, their final project -- a comedy about nerds learning martial arts, teaching their hard-nosed karate sensei a few life lessons in return -- won't ever see the light of day.

Heidi says the film was stalled in pre-production due to numerous issues, including finances.

The director also tells us she is still in shock and heartbroken over James' brutal slaying, which cops say came at the hands of his girlfriend's son. She recalls James talking about his partner often and how much he adored her.

The suspected killer is 44-year-old Michael Gledhill ... he's been arrested and booked into jail for murder. He's being held on a $2 million bond. Cops have not released a motive.